1. Field of the Invention
The present invention relates to a stencil-producing apparatus for producing stencils in a heat-sensitive medium using a plurality of thermal elements of, for example, a thermal head, and more particularly to the stencil-producing apparatus also serving as a printing apparatus.
2. Description of the Related Art
A conventional stencil-producing apparatus includes a thermal head containing a plurality of thermal elements. The stencil-producing apparatus forms stencils in a heat-sensitive stencil paper. The heat-sensitive stencil paper is made from a thermoplastic resin film adhered to a porous support member. The stencil-producing apparatus produces stencils by melting small holes, or perforations, in the thermoplastic resin film side of the heat-sensitive stencil paper using the thermal elements. As shown in FIG. 1, a conventional stencil-producing apparatus 100 includes the thermal head 25 containing the thermal elements 26, a platen roller 24 and transport rollers 22 and 23. The transport rollers 22 and 23 transport a sheet of heat-sensitive stencil paper 30 sandwiched therebetween in a direction indicated by arrow A (which will be referred to as an "auxiliary scanning direction," hereinafter) so as to insert it between the platen roller 24 and the thermal head 25. The platen 24 presses the thermoplastic resin film 30a of the heat-sensitive stencil paper 30 into direct contact with the thermal elements 26 of the thermal head 25. A stencil pattern is formed by perforations in the thermoplastic resin film 30a of the heat-sensitive stencil paper 30 by selectively heating the thermal elements 26.
When the thermal element 26 is energized and begin to heat, the temperature of the thermoplastic resin film 30a in direct contact with the thermal element 26 also rises. When the temperature of the thermoplastic resin film 30a rises to a predetermined shrivel start temperature ta (temperature where the thermoplastic resin film 30a starts to shrivel), a small perforation opens and expands in the thermoplastic resin film 30a. On the other hand, when energizing of the thermal elements 26 stops, the thermal elements 26 release heat and the temperature of the thermoplastic resin film 30a lowers below another predetermined shrivel stop temperature tb (temperature where shriveling stops) whereupon the perforation in the thermoplastic resin film 30a stops expanding and the thermoplastic film resin hardens. Incidentally the shrivel start temperature ta is greater than the shrivel stop temperature tb.
As shown in FIG. 2, when producing a stencil, each thermal element 26 of a size (a) melts a perforation dot having a size (b) in the heat-sensitive stencil paper 30, the size (b) being only slightly larger than the size (a). However, when stencil printing is performed using the perforation dot of the size (b), ink flows through the perforation dot of the size (b) and spreads on the print paper into a size (c) print pattern, the size (c) being much larger than the size (b). Therefore, although the size of the perforation in the heat-sensitive stencil paper 30 is almost the same as that of the thermal element 26 of the thermal head 25, when heat-sensitive stencil paper 30 with perforations of this size are used for perforation-stencil printing, the extent that ink spreads on the paper is rather large.
As the thermal head 25 of the stencil-producing apparatus 100 of FIG. 1, a thermal head employed in a conventional facsimile machine can be utilized. As shown in FIG. 4 (a), the thermal head 25 generally includes a plurality of thermal elements 26 arranged in a linear array in a main scanning direction B which extends perpendicularly to the auxiliary scanning direction A. Each of the thermal elements 26 has a rectangular shape having a width W in the main scanning direction B and having a length L in the auxiliary scanning direction A. The length L has a value almost twice the value of the width W. The thermal elements 26 are arranged in the main scanning direction B with a pitch P. The pitch P is slightly larger than the width W of the thermal element 26. A small amount of gap G is therefore formed between adjacent thermal elements 26. A pair of electrodes 27 are connected to both sides of each thermal element 26 in the auxiliary scanning direction A for supplying power to the thermal element 26. The transport rollers 22 and 23 are so designed to feed the heat-sensitive stencil paper 30 in the auxiliary scanning direction A by a line distance almost equal to the pitch P.
Such a thermal head 25 employed in a conventional facsimile machine is, however, not well suited for producing stencils in heat-sensitive stencil paper 30, as follows.
In the case where it is desired to produce a solid pattern stencil in the heat-sensitive stencil paper 30, all of the thermal elements 26 of the thermal head 25 are energized to heat the heat-sensitive stencil paper 30. Since the area (=W.times.L) of each thermal element 26 is relatively large, each thermal element 26 can apply high printing energy per unit area to the heat-sensitive stencil paper 30. As a result, the surface temperature at inter-dots areas between dots adjacent on the thermoplastic resin film 30a in the main scanning direction sometimes rises above the shrivel stop temperature tb. When this happens, the perforation enlarges from the center of each dot into the inter-dot space. Since the gap G between adjacent thermal elements 26 in the main scanning direction B is relatively small, the perforation continues enlarging into the adjacent dot. If many adjacent dots are connected in this way, a continuous perforation is formed in the main scanning direction, as shown in FIG. 4(b).
The auxiliary scanning operation by the transport rollers 22 and 23 feeds the heat-sensitive stencil paper 30 by the line distance P. The auxiliary scanning operation therefore arranges the dot perforations in the auxiliary scanning direction with the pitch P. Since the pitch P is smaller than the length L of each thermal element 26 and since the size of each dot perforation is almost the same as the size of the thermal element 26, the dot perforations arranged in the auxiliary scanning direction partly overlap with each other. Many adjacent dots are thus overlapped in this way, resulting in that a continuous perforation is formed also in the auxiliary scanning direction, as also shown in FIG. 4(b).
Attempting to produce such a solid stencil pattern in the heat-sensitive stencil paper 30 may therefore form a large continuous perforation with no inter-dot spaces, either in the main scanning direction or the auxiliary scanning direction. In this situation, melted and fluid thermoplastic resin film becomes entwined with the fiber of the support member, filling the pores therein. Ink can not pass through the support member when pores are clogged in this way and such clogged areas show up in the stencil print as white patches in black image portions. That is, there has been a problem in that printed images appear similar to those printed on traditional Japanese paper. Also, more ink is transferred to the print paper through areas with perforations connected as described above than through independent perforations, which increases the likelihood of set off. Also, areas of connected perforations can also be formed as described above when producing stencils of characters or lines because these are formed by dots in continuous horizontal and vertical rows, that same was as solid patterns. A large amount of ink is transferred to the print paper through areas in the film with connected perforations, creating blurred character images and line images because they print thicker than desired.
In order to solve the above-described problems, Japanese Patent Application Kokai No. HEI-2-67133 has proposed a thermal head suited for a stencil-producing apparatuses. This document has proposed to shorten the length L of each thermal element 26. That is, as shown in FIG. 5 (a), each thermal element 26 of the thermal head 25 of this document has a new length L' which is smaller than the pitch P. Since the heat-sensitive stencil paper 30 is fed in the auxiliary scanning direction by the line distance P, the dots produced to be arranged in the auxiliary scanning direction A do not overlap with each other. In addition, since the length L' is smaller than the length L, the area (W.times.L') of the proposed thermal element 26 becomes smaller than the area (W.times.L) of the thermal element 26 of FIG. 4(a). Accordingly, the printing energy per unit area applied from each thermal element 26 to the heat-sensitive stencil paper 30 becomes small, which prevents the perforation from enlarging from the center of each dot into the inter-dot space in the main scanning direction B and further into the adjacent dot.
Thus, the construction proposed by this document provides a space between dot perforations formed in the thermoplastic resin film 30a of the heat-sensitive stencil paper 30 both in the auxiliary scanning direction A and in the main scanning direction B. As shown in FIG. 5 (b) , when a solid pattern stencil is produced in heat-sensitive stencil paper 30 using this thermal head 25, each perforation dot is separate and unconnected in both the main scanning direction and the auxiliary scanning direction. Stenciling with perforation dot in this condition can form a good solid print.